I have a Sterling Corsair kit that is 10 years old and still looks new. I've also seen one year old kits that are trashed from care-
less storage: heat, humidity, mice, etc. The first thing that does
start showing it's age first, though, are any decals that might
come with the kit. If you are planning long-term storage, remove
the kit's decals and preserve them between acid-free paper rolled
up in a PVC tube sealed at both ends. I have water transfer decals from the 70's that were kept this way and they still work!
Here's a shot of the 10 year old corsair kit, that I plan to finally build this winter!